The fluidized catalytic cracking process is one of the major refining processes which is currently employed in the conversion of petroleum to desirable fuels such as gasoline and diesel fuel. In this process, a high molecular weight hydrocarbon feedstock is converted to lower molecular weight products through contact with hot, finely-divided, solid catalyst particles in a fluidized or dispersed state. Suitable hydrocarbon feedstocks typically boil within the range of from about 205.degree. C. to about 650.degree. C., and they are usually contacted with the catalyst at temperatures in the range from about 450.degree. C. to about 650.degree. C. Suitable feedstocks include various mineral oil fractions such as light gas oils, heavy gas oils, wide-cut gas oils, vacuum gas oils, kerosenes, decanted oils, residual fractions, reduced crude oils and cycle oils which are derived from any of these as well as fractions derived from shale oils, tar sands processing, and coal liquefaction. Products from a fluidized catalytic cracking process are typically based on boiling point and include light naphtha (boiling between about 10.degree. C. and about 221.degree. C.), heavy naphtha (boiling between about 10.degree. C. and about 249.degree. C.), kerosene (boiling between about 180.degree. C. and about 300.degree. C.), light cycle oil (boiling between about 221.degree. C. and about 345.degree. C.), and heavy cycle oil (boiling at temperatures higher than about 345.degree. C.).
Not only does the fluidized catalytic cracking process provide a significant part of the gasoline pool in the United States, it also provides a large proportion of the sulfur that appears in this pool. The sulfur in the liquid products from this process is in the form of organic sulfur compounds and is an undesirable impurity which is converted to sulfur oxides when these products are utilized as a fuel. These sulfur oxides are objectionable air pollutants. In addition, they can deactivate many of the catalysts that have been developed for the catalytic converters which are used on automobiles to catalyze the conversion of harmful engine exhaust emissions to gases which are less objectionable. Accordingly, it is desirable to reduce the sulfur content of catalytic cracking products to the lowest possible levels.
The sulfur-containing impurities of straight run gasolines, which are prepared by simple distillation of crude oil, are usually very different from those in cracked gasolines. The former contain mostly mercaptans and sulfides, whereas the latter are rich in thiophene, benzothiophene and derivatives of thiophene and benzothiophene.
Low sulfur products are conventionally obtained from the catalytic cracking process by hydrotreating either the feedstock to the process or the products from the process. The hydrotreating process involves treatment with hydrogen in the presence of a catalyst and results in the conversion of the sulfur in the sulfur-containing impurities to hydrogen sulfide, which can be separated and converted to elemental sulfur. Unfortunately, this type of processing is typically quite expensive because it requires a source of hydrogen, high pressure process equipment, expensive hydrotreating catalysts, and a sulfur recovery plant for conversion of the resulting hydrogen sulfide to elemental sulfur. In addition, the hydrotreating process can result in an undesired destruction of olefins in the feedstock by converting them to saturated hydrocarbons through hydrogenation. This destruction of olefins by hydrogenation is usually undesirable because it results in the consumption of expensive hydrogen, and also because the olefins are valuable as high octane components of gasoline. As an example, naphtha of a gasoline boiling range from a catalytic cracking process has a relatively high octane number as a result of a large olefin content. Hydrotreating such a material causes a reduction in the olefin content in addition to the desired desulfurization, and the octane number of the hydrotreated product decreases as the degree of desulfurization increases.
U.S. Pat. No. 2,448,211 (Caesar et al.) discloses that thiophene and its derivatives can be alkylated by reaction with olefinic hydrocarbons at a temperature between about 140.degree. and about 400.degree. C. in the presence of a catalyst such as an activated natural clay or a synthetic adsorbent composite of silica and at least one amphoteric metal oxide. Suitable activated natural clay catalysts include clay catalysts on which zinc chloride or phosphoric acid have been precipitated. Suitable silica-amphoteric metal oxide catalysts include combinations of silica with materials such as alumina, zirconia, ceria, and thoria. U.S. Pat. No. 2,469,823 (Hansford et al.) teaches that boron trifluoride can be used to catalyze the alkylation of thiophene and alkyl thiophenes with alkylating agents such as olefinic hydrocarbons, alkyl halides, alcohols, and mercaptans. In addition, U.S. Pat. No. 2,921,081 (Zimmerschied et al.) discloses that acidic solid catalysts can be prepared by combining a zirconium compound selected from the group consisting of zirconium dioxide and the halides of zirconium with an acid selected from the group consisting of orthophosphoric acid, pyrophosphoric acid, and triphosphoric acid. The Zimmerschied et al. reference also teaches that thiophene can be alkylated with propylene at a temperature of 227.degree. C. in the presence of such a catalyst.
U.S. Pat. No. 2,563,087 (Vesely) discloses that thiophene can be removed from aromatic hydrocarbons by selective alkylation of the thiophene and separation of the resulting thiophene alkylate by distillation. The selective alkylation is carried out by mixing the thiophene-contaminated aromatic hydrocarbon with an alkylating agent and contacting the mixture with an alkylation catalyst at a carefully controlled temperature in the range from about -20.degree. C. to about 85.degree. C. It is disclosed that suitable alkylating agents include olefins, mercaptans, mineral acid esters, and alkoxy compounds such as aliphatic alcohols, ethers and esters of carboxylic acids. It is also disclosed that suitable alkylation catalysts include the following: (1) the Friedel-Crafts metal halides, which are preferably used in anhydrous form; (2) a phosphoric acid, preferably pyrophosphoric acid, or a mixture of such a material with sulfuric acid in which the volume ratio of sulfuric to phosphoric acid is less than about 4:1; and (3) a mixture of a phosphoric acid, such as orthophosphoric acid or pyrophosphoric acid, with a siliceous adsorbent, such as kieselguhr or a siliceous clay, which has been calcined to a temperature of from about 400.degree. to about 500.degree. C. to form a silico-phosphoric acid combination which is commonly referred to as a solid phosphoric acid catalyst.
U.S. Pat. No. 5,171,916 (Le et al.) is directed to a process for upgrading a light cycle oil by: (1) alkylating the heteroatom containing aromatics of the cycle oil with an aliphatic hydrocarbon having at least one olefinic double bond through the use of a crystalline metallosilicate catalyst; and (2) separating the high boiling alkylation product by fractional distillation. It is disclosed that the unconverted light cycle oil has a reduced sulfur and nitrogen content, and the high boiling alkylation product is useful as a synthetic alkylated aromatic functional fluid base stock.
U.S. Pat. No. 5,599,441 (Collins et al.) discloses a process for removing thiophenic sulfur compounds from a cracked naphtha by: (1) contacting the naphtha with an acid catalyst in an alkylation zone to alkylate the thiophenic compounds using the olefins present in the naphtha as an alkylating agent; (2) removing an effluent stream from the alkylation zone; and (3) separating the alkylated thiophenic compounds from the alkylation zone effluent stream by fractional distillation. It is also disclosed that additional olefins can be added to the cracked naphtha to provide additional alkylating agent for the process.